Beauty With Brains

Butterflies have long enchanted us with their good looks; now scientists are discovering that the insects' abilities are even greater than we think

04-01-2004
//
Doug Stewart

WATCHING A BUTTERFLY zigzag aimlessly across a meadow on a sunny morning, you could easily take it for nature’s most carefree vagabond—unhurried, unburdened, even a little ditzy. You’d be mistaken. Butterflies are purposeful, aggressive, sexually driven and smarter than you think. Most of us, however, enchanted as we are by the butterfly’s flamboyant good looks and rollicking flight path, are unaware of these traits. That’s understandable, says Georgetown University biologist Martha Weiss, an expert on butterfly behavior.

“Unlike honeybees, which have a reputation as the intellectuals of the insect world,” Weiss says, “butterflies spend their time basking in the sun and sipping nectar from beautiful flowers. It’s easy to think they’re a bit indolent.”

New research on butterflies is proving that these insects are capable of an astonishing range of clever behaviors, from thwarting attacks to outwitting competitors, from learning lessons to navigating long distances. “They don’t have a lot of gray matter in their brains, maybe just a cubic millimeter,” says Weiss, “but with it they can do everything they need to do.”

Much of a butterfly’s behavior is hard-wired, of course, not learned by trial and error. Consider the caterpillar. “The lifestyle of all insect larvae is risky,” says long-time butterfly researcher Thomas Eisner, a professor of chemical ecology at Cornell University. “They can’t fly. They’re slow. So they have a lot of interesting strategies to defend themselves.”

When attacked, some caterpillars pop foul-smelling tentacles, or osmeteria, from their thoraxes and brandish them menacingly. Young swallowtail caterpillars disguise themselves as bird droppings (not very scary, perhaps, but distinctly inedible). In a later molt, the same caterpillars can resemble snakes—eyes, patterning and all. “Some caterpillars feed on toxic plants, then they vomit,” Eisner says. “It’s a way to use the plant’s toxins for their own defense.” Other caterpillars, notably the monarch, incorporate the toxins they consume in their own tissues, rendering them poisonous to predators even as adults; a bird takes a bite and immediately drops the caterpillar, which often survives the encounter.

Caterpillars of the European cabbage butterfly, now common in North America, make a poison of their own, a toxin that Eisner’s research team recently discovered. “The caterpillar has droplets of this chemical on the tips of the hairs on its back,” he says. “Ants immediately back off and begin cleansing any part of the body that came in contact with it.”

Caterpillars go through a series of molts as they grow, shedding their bodies’ stiff outer layer at each stage. Metamorphosis, when the larva transforms itself into one of nature’s most charming aerialists, can be thought of as an extreme form of molting.

As a chrysalis, its rigidly encased final stage of pre-adulthood, the animal isn’t hibernating. It’s remaking itself furiously. It emerges after a week or so with huge compound eyes, long segmented legs, oversized wings and newly reorganized muscles to power them. Caterpillars can live for months, while most adult butterflies survive just a week or two. In any one species, however, life expectancy depends on the season in which an egg hatches and how much food is available. Overwintering butterflies can live six months or, in a few Arctic and Alpine species, a year or longer.

Butterflies have highly specialized mouthparts, with a long tubelike proboscis that uncoils like a garden hose to sip sugary flower nectar—an insect’s version of aviation fuel. They also have excellent color vision, useful for identifying both nectar-rich flowers and host plants for egg-laying. Using fake flowers made of construction paper, Weiss has trained butterflies of several species—monarch, pipevine swallowtail and gulf fritillary—to associate a particular color with a sugar-water reward. “Some butterflies will pick it up in one bout,” she says, though four or five tries is more typical. Flowers are a highly variable resource, she points out, so a creature depending on them for survival needs to be flexible, especially outside its home range. “If an insect with a hardwired preference for blue ended up in a meadow with only yellow and pink flowers, it would be in trouble.”

Butterflies help pollinate many flowering plants, even if more ostentatiously hardworking bees and wasps get the credit for this. The yellow-fringed orchid (Platanthera ciliaris) found throughout the eastern United States is pollinated only by a swallowtail butterfly. “The flowers have a very long nectar spur that only a long-tongued butterfly can get all the nectar out of,” says Don Harvey, a lepidopterist at the Smithsonian’s National Museum of Natural History. Rather than dusting the visitor with pollen grains, the orchid sticks a whole packet of pollen onto the swallowtail’s body. The insect then transfers it—precisely if unintentionally—to just the right spot on the next orchid.

Learning Curve

Uniquely, butterflies of the tropical genus Heliconius (which in the United States includes the zebra longwing and the gulf fritillary) have evolved a taste for pollen as a food, which supplements their nectar intake. As a result, the insects spend less time as a carbo-loading caterpillar bulking up for the future and more time as pollen-feeding adults, according to Lawrence Gilbert, a population biologist at the University of Texas–Austin. Over time, he says, natural selection has favored Heliconius>butterflies with longer life spans and more brainpower. He found that foraging Heliconius butterflies learn to follow precise routes from flower to flower, visiting the same plant at the same time, day after day. As old flowers close and new flowers open, the insects learn new routes. Younger butterflies have even been observed following older butterflies as though they were apprentices.

In Trinidad, Gilbert’s research team once marked the wings of local butterflies with easy-to-read ID numbers. “I watched a group of about 15 Heliconius females we’d marked visit the same clump of roadside flowers every morning,” he says. A road crew happened by one day and whacked down the flowers. “For the next five or six days, I saw the same butterflies show up at that spot at the same time. They’d look around like they were lost and thinking, ‘Hey, where are those flowers?’”

Legally Blind

Further evidence of the insects’ learning ability: Heliconius butterflies that used to escape from Gilbert’s greenhouses in Austin would behave strangely. “They would fly out the door into the open,” he says, “and they’d fly only as high as the roof and back down again. They seemed to learn the dimensions of the greenhouse as part of their home range. They never bashed their wings on the walls like most butterflies.” They also seemed to memorize the location of spiderwebs; butterflies new to a greenhouse were most likely to be snared. In the wild, Heliconius butterflies will even avoid spots where in the past they’ve been netted.

Gilbert has evidence that these butterflies use visual landmarks, not scent or other signals, to plot their routes. Whatever landmarks they use, they are probably big ones. The compound eyes of butterflies are large and impressive, but the insects are nearsighted all the same. “By human standards, butterflies have 20

1,000 vision, which means they’re legally blind,” says Ron Rutowski, a behavioral ecologist at Arizona State University. The butterflies that size up one another notice brightness and movement but probably not much visual detail. This would explain why males occasionally court a dead leaf fluttering in the wind.

When one’s adulthood is measured in weeks, finding a mate is obviously a matter of some urgency. Males take the initiative. In some Heliconius species, they save time by perching on a female’s chrysalis and copulating with her as she emerges. During mating, males pass a spermatophore, a large case containing sperm and nutrients, to the female as a sort of paternal gift. Some females may rely on the nutrients for the draining chore of egg-production. To produce this costly gift, males of some species have to round up salts and nitrogen compounds not found in nectar. This explains the bizarre phenomenon known as puddling, in which male butterflies congregate by the thousands to insert their proboscises into patches of muddy ground. Mineral-starved males will also partake of carrion, dung and urine.

A male’s generosity is amply rewarded. The spermatophore may render his mate unreceptive to other males until she finishes digesting it, a process that can take a week. Some spermatophores reek with an anti-aphrodisiac odor that cools other males’ ardor. If a new suitor approaches anyway, the female may perform a rejection dance, flying straight up and swooping down again, or perhaps fluttering frantically in imitation of an excited male—a sure turnoff.

Frantic or sedate, the very act of fluttering is one of a butterfly’s most characteristic traits. Its wing beat is relatively slow for an insect. A monarch will flap its wings 5 to 12 beats a second, for example, while some flies beat their wings 1,000 times a second. A skipper can travel in bursts at nearly 50 miles an hour, approaching dragonfly speed, but most butterflies’ progress is slower and more erratic. Not that butterflies are clumsy: They’re masters of the pinpoint landing, alighting on flower petals with their tongues uncoiled and ready to drink.

Aerodynamically, butterflies have much larger wings than necessary for staying aloft. Their oversized wings, in fact, are responsible for their herky-jerky trajectory. Eisner believes the butterfly evolved its clumsy-looking flight style as a defense. “Their erratic flight tells birds, ÔI’m not worth pursuing. I’m too hard to catch, and I’m not much of a meal,’” he says. “Butterflies are colorful and visible at a great distance, but they’re all wrapper, no candy.” He believes birds end up discriminating against them as a group compared to other prey.

Warning Signs

In many cases, a butterfly’s giant wings serve as a none-too-subtle warning to predators that the meal they represent is not so much meager as unpalatable. The butterflies with the most colorful wings and leisurely flight styles—such as monarchs and zebra longwings—are the toxic ones. Tastier butterflies—red admirals, painted ladies and Mexican fritillaries—are apt to have bright topsides and dull undersides. “It’s the best of all possible worlds, isn’t it?” says Phil Schappert, a butterfly researcher at the University of Texas–Austin. “You can be bold when you need to,” signaling your identity to potential mates or rivals, for example, “or you can land and close your wings up and literally disappear.”

Butterflies don’t always flutter. Long-distance migrants like monarchs will save energy by holding their wings in a flat “V” and gliding. “When they’re migrating,” says Orley Taylor, a University of Kansas ecologist and director of Monarch Watch, a mark-and-recapture program, “monarchs will fly a few feet off the ground in the morning until they hit a thermal rising off some barren earth or asphalt. Then they’ll rise like a hawk.” Monarchs can ride a thermal to 5,000 feet. Out of sight of humans, they migrate in such dense concentrations on their way to their usual wintering grounds in Mexico each fall that they show up on radar. Despite flying 2,000 miles on paper-thin wings, their mastery of low-powered flight means they’re barely winded when they get there. “A lot of monarchs arrive in remarkable condition,” says Taylor. “They look like they just hatched.”

On the Fly

Migration in butterflies is triggered by environmental conditions, not a genetic compulsion, so members of a single species can behave in widely different ways. Monarchs in the United States that emerge in mid-summer don’t migrate at all. They’re too short-lived to worry about winter. Those emerging in late summer and early fall are the ones that head south.

“The interesting thing,” says Taylor, “is that a monarch from Maine, one from Minnesota and one from Wyoming will all converge on a fairly narrow area, a set of mountaintops west of Mexico City.” They’re not following a compass direction because each insect takes a different heading to get there. “What that means is that butterflies can tell longitude, which is remarkable.”

Taylor has flown from Kansas to Washington, D.C., with monarchs in his carry-on luggage to see which way the insects would try to migrate when they got there. “For about 48 hours, they still think they’re in Kansas. Then they get acclimated and pick up the new orientation that the local butterflies have.” He suspects they sense a mix of local environmental cues, including the Earth’s magnetic field, to chart their course.

“If you think about it, this is what migrating butterflies have to do whenever they’re blown off course. They have to reorient themselves.” The details of how they do it remain a mystery, but somehow those little butterfly brains have it all worked out.

“Butterflies add another dimension to the garden,” wrote British naturalist Miriam Rothschild, “for they are like dream flowers—childhood dreams—which have broken loose from their stalks and escaped into the sunshine.”

Animating your yard with these delightful creatures doesn’t require much effort or expertise. If you plant the nectar and larval plants they need, butterflies will find them.

Butterfly-friendly perennials include asters, phlox, goldenrod, milkweed, coreopsis, dianthus, blazing star and joe-pye weed. Annuals include nasturtium, marigolds, cosmos and zinnias. The butterfly bush (Buddleia davidii), which can grow to 10 feet, is a butterfly magnet, but beware: It is a nonnative species that can become invasive.

To keep the butterflies coming, choose a mix of flowering plants that blossom from early spring through late fall. What to plant depends on where you live. Butterflies are happiest flitting about gardens with lots of sun and not much wind. Expanses of rock, brick or gravel bathed in morning sunlight let them start the day by boosting their body temperature. An open patch of wet sand or mud near the garden encourages “puddling” by mineral-hungry males. Mixing in a little manure or salt is even better, provided you keep any salt away from your flowers’ roots.

To make your garden even more irresistible to butterflies, plant host plants for their caterpillars, too. Larvae are pickier eaters than nectar-sipping adults, and females lay eggs only on plant species that their future offspring can eat—without these plants, the eggs will hatch but the caterpillars will starve or be poisoned. If you fancy monarchs, plant milkweed. For black swallowtails, grow parsley or dill. Checkered skippers go for hollyhocks.

The plants that caterpillars prefer are sometimes less attractive than the usual garden-center showboats. “It’s best if you don’t try to be too tidy and obsessive,” says long-time butterfly gardener Claire Hagen Dole, editor of The Butterfly Gardener’s Guide, published by the Brooklyn Botanic Garden. “And you have to be willing to tolerate a bit of leaf damage.” In most cases, birds, wasps, lizards and other predators will keep your plants from being defoliated.

“Don’t freak out if some of your caterpillars fall prey to predators,” says Dole. “If you like birds, keep in mind that caterpillars are a great protein source for them. The more diverse the population in your garden, the healthier it’s going to be.”

Butterfly Basics

World’s smallest: Pygmy blue found in southern California, with a wingspan of just more than a half inch

World’s largest: New Guinea’s Queen Alexandra’s birdwing measures up to 12 inches from wingtip to wingtip

Range: Worldwide except Antarctica

Strength: Caterpillars have more than 1,000 muscles, which they use to move from place to place, often at a surprisingly quick pace

Biggest threats: Habitat loss, pollution and insecticides

Stages: A caterpillar passes through about five stages, called instars, in two or three weeks, shedding its exoskeleton and becoming larger with each stage